This invention pertains to the art of impact arrangements for casks or packages for shipping radioactive waste.
Two categories of radioactive waste casks are called type A and type B. Each category has requirements for withstanding certain impact forces and other conditions such as internal and external heat, for example. Type B casks, with which this application mainly deals, must be capable of withstanding impacts delivered by dropping a cask 30 feet upon a substantially unyielding surface with the cask having the following orientations in the drops:
flat top end or bottom end drop PA1 top or bottom end corner drop PA1 side drop.
One cask end arrangement for absorbing impact energy is disclosed in U.S. Pat. No. 4,268,755. This patent discloses a fuel assembly shipping cask including "a shock-absorbing piston member associated with the underside of the transfer cask, the piston member comprising two mutually spaced-apart metal plates, a multiplicity of hollow metal bodies stacked on top of one another therebetween and fastening elements for fastening the piston member to the lifting devices. If the transfer cask should then fall, a deformation of the hollow bodies would result. Energy would consequently be consumed, so that the forces generated upon impact to the cask would not be merely temporarily but entirely reduced. The following transfer cask would thus be gently decelerated and would not rebound elastically . . . it is essential that a material with relatively great deformation energy be used which can experience or withstand a great amount of plastic elongation or expansion before it breaks, and that, through the herein afore-mentioned cavities, space is provided for this plastic elongation or expansion."
U.S. Pat. No. 3,675,746 discloses a deformable impact energy absorber for use with spent nuclear reactor fuel shipping casks. The absorber includes a large diameter tubulation 12 within which is packed a plurality of smaller tubes 14. It is said "as the cross-sectional area of the tubulation 12 decreases, the resistance to deformation increases due to progressive deformation of the tubes 14. This increase in resistance to deformation increases at a readily predictable and somewhat linear rate with deformations, as shown in FIGS. 3 and 4. The tubulation can contain stainless steel or other ductile, high strength steels, metal or alloys."
U.S. Pat. No. 4,423,802 discloses a cask end cap which includes a number of different compartmentalized spaces formed by sheet metal members with part of the compartments containing soft dampening materials such as balsa wood, and the other compartments containing harder dampening material such as hard wood.
It is my view that these approaches to absorbing impact energy by the use of too easily crushable arrangements as disclosed, as well as the use of foam materials and honeycomb structures, are inferior to my approach in a number of respects. With such materials and structures, pressure and energy absorption increase as a function of displacement. A major fraction of the energy is absorbed during the final stages of crushing when the pressure is the highest. This results in high deceleration forces and deceleration.
The aim of this invention is to provide impact absorbing means for a cask in which the deceleration is as relatively constant as is consistent with the constraints of geometry of the impact absorbing means.